Prefer - Professionals you need, from people you know
Linphone - Linphone is an internet phone or Voice Over IP phone (VoIP).
Warmspace - Tech-powered conversations that transform relationships in work and life.
Zoiper - Cross-platform VoIP softphone dialer for SIP and IAX systems
Introwise - All-in-one scheduling and business management platform for coaches, tutors and consultants to run their business online.
Sipdroid - Free and open-source SIP/VoIP Client